As is well known, photoconductive substances for electrophotographic light-sensitive compositions include many organic compounds. Some of these organic compounds have been confirmed to have considerably high sensitivity. In fact, however, only a very limited number of organic photoconductive substances have been used in the preparation of electrophotographic materials.
Organic photoconductive substances have many superior properties compared with inorganic photoconductive substances, and therefore, may find many uses in the field of electrophotography. For example, transparent electrophotographic light-sensitive films, flexible electrophotographic light-sensitive films, those electrophotographic light-sensitive films which are light and easy to handle, and so forth can be prepared very advantageously using organic photoconductive substances. In addition, organic photoconductive substances have characteristics which could not be expected from the characteristics inorganic photoconductive substances e.g., film-forming properties during the production of electrophotographic light-sensitive materials, surface smoothness, and choice of the desired charging polarity in electrophotographic processes.
These organic photoconductive substances, however, have not yet been widely used in the field of electrophotography mainly because of their low sensitivity and poor film strength, although they have many excellent characteristics as described above.
Initially, low molecular heterocyclic compounds, nitrogen-containing aromatic compounds, various polymer type aromatic compounds, etc. have been extensively investigated as organic photoconductive substances. However, the sensitivity of these compounds must be increased to much higher levels, because even those organic photoconductive substances which are known to have the highest sensitivity do not have sufficient sensitivity to be put into practical use as such without the application of a sensitization treatment. For this reason, various sensitization methods have been studied. In the practical use of organic photoconductive substances, therefore, it is essential to select and employ a sensitization method which is most effective for them. It may safely by said that the industrial value of organic photoconductive substances is determined by the sensitization technique to be employed, i.e., by the increased sensitivity of the ultimate electrophotographic light-sensitive material which is achieved by application of the sensitization technique.
Addition of Lewis acids is such a sensitization method. In this method, sensitization is achieved by the appearance of a new spectral sensitivity resulting from the formation of a donor/acceptor complex between Lewis acids and organic photoconductive substances.
All the systems comprising organic photoconductive substances, sensitizing dyes, and heretofore known Lewis acids have poor stability with the lapse of time and are not suitable for practical use.